We have found that the lungs of patients with active pulmonary fibrosis contain connective tissue cells reacting with monoclonal antibodies directed against the carboxylterminal propeptide domain of type I collagen, although healthy lungs lacked these cells entirely. We have also found that this altered phenotype which is characteristic of fibroblasts in embryonic or healing wounds, is reproduced in the dermis following injection of pure transforming growth factor-beta (TGF-beta). Thus, pulmonary fibrosis is associated with the proliferation of connective tissue synthesizing cells - "fibroblasts" - as well as with alterations in the collagen synthesizing phenotype of fibroblasts. We propose to test the hypothesis that certain "growth factors", such as TGF- beta and platelet derived growth factor are responsible for expanding the fibroblast population and modulating their phenotype by increasing the expression of fibronectin and collagen at sites of lung injury. We will also try to determine whether there are subpopulations of lung fibroblasts which may respond differently to these mediators in vivo. We will use sensitive immunohistologic, biochemical and molecular biologic methods to examine the effects of these "growth factors" on fibroblast differentiation. Monoclonal antibodies to type I procollagen and other connective tissue molecules, as well as nucleic acid probes for fibronectin and the interstitial and basement membrane collagens and growth factors will be employed. These methods should allow us to assess the connective tissue synthesizing phenotype of individual human lung fibroblasts in intact tissues and cell culture, and provide evidence for their origins. Studies of intact tissues are always difficult, but are ultimately essential for understanding the complex cellular and molecular events associated with the response of cells to injury in vivo. In vitro studies of cell populations usually obscure cellular heterogeneity and provide no more than an average assessment of differentiated function. By contrast, the approaches outlined in this application should permit a direct assessment of cellular differentiation and lung fibroblast heterogeneity with respect to their most fundamental function, i.e. the synthesis and deposition of extracellular matrix.